Vertical transistor with horizontal gate layers

ABSTRACT

Vertical body transistors with adjacent horizontal gate layers are used to form a memory array in a high density flash electrically erasable and programmable read only memory (EEPROM) or a logic array in a high density field programmable logic array (FPLA). The transistor is a field-effect transistor (FET) having an electrically isolated (floating) gate that controls electrical conduction between source regions and drain regions. If a particular floating gate is charged with stored electrons, then the transistor will not turn on and will provide an indication of the stored data at this location in the memory array within the EEPROM or will act as the absence of a transistor at this location in the logic array within the FPLA. The memory array or the logic array includes densely packed cells, each cell having a semiconductor pillar providing shared source and drain regions for two vertical body transistors that have control gates overlaying floating gates distributed on opposing sides of the semiconductor pillar. Both bulk semiconductor and silicon-on-insulator embodiments are provided. If a floating gate transistor is used to store a single bit of data or to represent a logic function, an area of only 2F 2  is needed per respective bit of data or bit of logic, where F is the minimum lithographic feature size.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to integrated circuits and inparticular to floating gate transistors.

[0003] 2. Description of the Related Art

[0004] Programmable memories and logic circuits are integral parts of adigital system, such as a computer, and can have similar physicalstructures. One type of programmable memories is an electricallyerasable and programmable read only memory (EEPROM), which is areprogrammable nonvolatile memory that is widely used in the computersystem for storing data both when power is supplied or removed. Thetypical data storage element of an EEPROM is a floating gate transistor,which is a field-effect transistor (FET) having an electrically isolated(i.e., floating) gate that controls electrical conduction between sourceand drain regions. Data is represented by charges stored on the floatinggate and the resulting conductivity obtained between the source regionand the drain region.

[0005] Recently, logic circuits based on programmable memories have beenintroduced. The logic circuits are implemented by field programmablelogic arrays (FPLAs) which provide a flexible architecture viauser-programmed on-chip fuses (i.e., switches) to perform specificfunctions for a given application. The FPLAs are fabricated withfloating gate transistors similar to the programmable memories. However,the floating gate transistors in the FPLAs act as switches rather thanas storage elements. A common problem among the programmable memoriesand the related FPLAs is a large cell size, which limits circuitdensity.

SUMMARY OF THE INVENTION

[0006] The present invention provides floating gate transistors withvertical bodies and horizontal gates stacked next to the correspondingvertical bodies. In one embodiment, the floating gate transistorincludes a pillar of semiconductor material extending outwardly from aworking surface of a substrate to form a source region, a body regionand a drain region of the floating gate transistor. A floating gate isformed along one side of the pillar, and a control gate overlays thefloating gate. In one embodiment, the pillar is formed by etching aspart of the first steps in fabricating the floating gate transistor.

[0007] Electronic charges are selectively stored in the floating gate inprogramming the floating gate transistor. An absence or presence ofstored charges on the floating gate determines a conductivity state ofthe transistor between the source region and the drain region. In oneembodiment, hot electron injection is used to program the floating gatetransistor. In an alternate embodiment, Fowler-Nordheim tunneling isused to program the floating gate transistor.

[0008] In one embodiment, floating gate transistors form an array. Thearray includes a plurality of semiconductor pillars arranged in rows andin columns. The pillars form respective bodies of the floating gatetransistors. A plurality of floating gates forms in trenches between thecolumns of pillars, and the floating gates are separated from respectivesides of the pillars by a gate dielectric. A plurality of control gatesoverlay the respective floating gates, and the control gates areseparated from the respective floating gates by an intergate dielectric.

[0009] In one embodiment, the pillars are etched as part of an initialfabrication step to extend vertically from a substrate. For example,each body of the respective floating gate transistors extend outwardlyfrom the substrate with a source region formed proximally to thesubstrate, a body region above the source region, and a drain regionabove the body region.

[0010] In one embodiment, two floating gates lie adjacent to each otherin each trench between the columns of semiconductor pillars, and onecontrol gate overlays the adjacent floating gates. In an alternateembodiment, one floating gate lie in each trench between the columns ofsemiconductor pillars, and one control gate overlays the floating gate.In another embodiment, two floating gates lie adjacent to each other ineach trench between the columns of the semiconductor pillars, and twocorresponding control gates lie adjacent to each other above thefloating gates.

[0011] In one embodiment, an array of floating gate transistors is amemory cell array with the source regions of common rows electricallyconnected to be first input selection lines, the control gateselectrically connected along the direction of the columns to be secondinput selection lines, and the drain regions of common columnselectrically connected to be output data lines. In an alternateembodiment, an array of floating gate transistors is a logic array withthe source regions of a common column electrically coupled to beselection lines during programming of the logic array, the control gateselectrically coupled along the direction of the columns to be inputs tothe logic array, and the drain regions of a common row electricallycoupled to be output lines of the logic array. In another embodiment, anarray of floating gate transistors is a field programmable logic arraywith the source regions of a common column electrically interconnected,the drain regions of a common row electrically interconnected, and thecontrol gates interconnected along the direction of the columns.

[0012] Charges stored in the floating gates of a memory cell arrayrepresent data of the memory cell array. In one embodiment, hot electroninjection is used to selectively place charges in the respectivefloating gates of the memory cell array, thereby writing data memory.

[0013] In one embodiment, a floating gate transistor is fabricated upona substrate. The floating gate transistor includes a first conductivitytype semiconductor pillar formed upon the substrate. The pillar has topand side surfaces. A first source/drain region of a second conductivitytype forms in a portion of the pillar that is proximal to an interfacebetween the pillar and the substrate. A second source/drain region of asecond conductivity type forms in a portion of the pillar that is distalto the substrate and is separated from the first source/drain region. Agate dielectric forms on at least a portion of one side surface of thepillar. A floating gate forms substantially adjacent to a portion of theside surface of the pillar and is separated therefrom by the gatedielectric. An intergate dielectric forms on a top surface of thefloating gate. A control gate substantially overlays the floating gateand is insulated therefrom by the intergate dielectric.

[0014] Electrical charges in the floating gate controls electricalconduction between the first source/drain region and the secondsource/drain region of the floating gate transistor. In one embodiment,the floating gate transistor is a data storage element in a programmablememory array with the data represented by charges stored in therespective floating gates.

[0015] In one embodiment, fabrication of the floating gate transistorsincludes using a sacrificial gate layer to define a gate length. Thesacrificial gate layer is selectively removed and replaced with afloating gate in a subsequent step. In one embodiment, the sacrificialgate layer is undoped oxide, and the floating gate is doped polysilicon.

[0016] In one embodiment, dopant layers are on top and bottomrespectively of a sacrificial gate layer. The dopant layers and thesacrificial gate layer form in a trench defined by pillars ofsemiconductor material which form source, body and drain regions oftransistors. In one embodiment, the sacrificial gate layer substantiallyaligns with the body region in the horizontal direction. The bottomdopant layer substantially aligns with an interface between the sourceand body regions, overlapping both regions in the horizontal direction.The top dopant layer substantially aligns with an interface between thedrain and body regions, overlapping both regions in the horizontaldirection.

[0017] In one embodiment of a fabrication process, heat treatment isused to form self-aligned transistor structures. For example, diffusionof the dopant layers during the heat treatment results in lightly dopedsource/drain regions in the body region. The lightly doped source/drainregions act as extensions of the source and drain regions respectively.The separation distance between the lightly doped source/drain regionsis partially controlled by the thickness of the sacrificial gate layer.When the floating gate replaces the sacrificial gate layer, minimal gateoverlaps occur with the lightly doped source/drain regions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a block diagram of a high-level organization of acomputer.

[0019]FIG. 2 is a block diagram illustrating one embodiment of anonvolatile memory, including an array having a plurality of memorycells.

[0020]FIG. 3 is a block diagram of a field programmable logic array.

[0021]FIG. 4 is a schematic diagram illustrating one embodiment of afield programmable logic array.

[0022]FIG. 5 is a schematic diagram illustrating one embodiment of aprogrammable decoder array.

[0023]FIG. 6 is a schematic diagram illustrating one embodiment of anarray of memory cells.

[0024]FIG. 7A illustrates a top view of a portion of an array havingsplit control gates.

[0025]FIG. 7B illustrates a front view of a portion of an array havingsplit control gates.

[0026]FIG. 8A illustrates a top view of a portion of an array having asingle control gate.

[0027]FIG. 8B illustrates a front view of a portion of an array having asingle control gate.

[0028]FIG. 9A is a perspective view of one embodiment of verticaltransistors with horizontal gate layers, illustrated by a portion of anarray having split control gates.

[0029]FIG. 9B is a cross-sectional view of the array of FIG. 9A lookingperpendicular to output lines OL1-OLN.

[0030]FIG. 10 is a plan view looking toward the working surface of asubstrate, illustrating one embodiment of logic cells.

[0031]FIG. 11 is a perspective view of an alternate embodiment ofvertical transistors with horizontal gate layers, illustrated by aportion of an array having a single control gate.

[0032]FIG. 12 is a perspective view of another embodiment of verticaltransistors with horizontal gate layers, illustrated by a portion of anarray having a single control gate overlaying split floating gates.

[0033]FIG. 13 illustrates a side view of a plurality of source/drainlayers on top of a substrate material for forming vertical transistors.

[0034]FIG. 14 illustrates a plurality of first troughs extending througha plurality of layers formed on top of a substrate material.

[0035]FIG. 15 is a perspective view of a plurality of second troughsorthogonal to the first troughs of FIG. 14.

[0036]FIG. 16 is a cross-sectional view looking in the direction of thesecond troughs of FIG. 15.

[0037]FIG. 17 is a cross-sectional view looking in the direction of thesecond troughs of FIG. 16, illustrating a barrier layer stripped fromthe sidewalls of the second troughs.

[0038]FIG. 18 is a cross-sectional view looking in the direction of thesecond troughs of FIG. 17, illustrating a conductive layer in the secondtroughs.

[0039]FIG. 19 is a perspective view of the second troughs of FIG. 18,illustrating spacers positioned for use as a mask.

[0040]FIG. 20 is a perspective view of the second troughs of FIG. 17,illustrating an alternate method of forming vertical transistors withhorizontal gate layers.

[0041]FIG. 21 is a cross-sectional view looking in the direction of thesecond troughs of FIG. 16, illustrating yet another method of formingvertical transistors with horizontal gate layers.

[0042]FIG. 22 is a cross-sectional view looking in the direction of thesecond troughs of FIG. 21, illustrating gate length control and aself-aligned transistor structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0043] In the following detailed description of the invention, referenceis made to the accompanying drawings which form a part thereof, and inwhich is shown, by way of illustration, specific embodiments in whichthe invention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views.

[0044] In the following description, the terms wafer and substrate areinterchangeably used to refer generally to any structure on whichintegrated circuits (ICs) are formed, and also to such structures duringvarious stages of integrated circuit fabrication. Both wafer andsubstrate include doped and undoped semiconductors, epitaxialsemiconductor layers supported by a base semiconductor or insulator,combinations of such layers, as well as other semiconductor structureswell known to one skilled in the art, including bulk semiconductor andsemiconductor-on-insulator (SOI) substrates. The term conductor isunderstood to include semiconductors, and the term insulator is definedto include any material that is less electrically conductive than thematerials referred to as conductors

[0045]FIG. 1 is a block diagram of a high-level organization of acomputer 10. A computer 10 is a system whose functional elements includean arithmetic/logic unit (ALU) 20, a control unit 30, a memory unit 40and input/output (I/O) devices 50. The ALU 20 and the control unit 30are parts of a central processing unit (CPU). The control unit 30controls the operations of the ALU 20, the I/O devices 50 and the memoryunit 40. The ALU 20 interacts with the I/O devices 50 and the memoryunit 40.

[0046] The memory unit 40 contains data plus a stored list ofinstructions, which can be retrieved and executed by the ALU 20. Thedata and instructions can be stored using nonvolatile high densityelectrically erasable and programmable read only memories (EEPROMs) thatallow simultaneous erasure of multiple data bits, referred to as flashEEPROMs.

[0047]FIG. 2 is a block diagram illustrating one embodiment of anonvolatile memory 100, including a memory cell array 105 having aplurality of memory cells. In the embodiment of FIG. 2, the nonvolatilememory 100 is a flash EEPROM. However, the invention can be applied toother semiconductor devices, such as static or dynamic random accessmemories (SRAMs or DRAMs, respectively), synchronous random accessmemories or other types of memories that include a matrix of selectivelyaddressable memory cells.

[0048] The nonvolatile memory 100 includes the memory cell array 105,having cells therein that include floating gate transistors, asdescribed below. An X gate decoder 115 provides a plurality of gatelines, XG1, XG2 . . . XGN for addressing the floating gate transistorsin the memory cell array 105, as described below. A Y source/draindecoder 120 provides a plurality of first source/drain interconnectionlines YS1, YS2 . . . YSN, for accessing first source/drain regions ofthe floating gate transistors in the memory cell array 105, as describedbelow. In an embodiment in which commonly connected first source/draininterconnection lines YS1, YS2 . . . YSN are used, the Y source/draindecoder 120 may be omitted. An X source/drain decoder 125 provides aplurality of data lines, XD1, XD2 . . . XDN for accessing secondsource/drain regions of the floating gate transistors in the memory cellarray 105, as described below. The X source/drain decoder 125 alsoincludes sense amplifiers and I/O circuitry for reading, writing, anderasing data to and from the memory cell array 105.

[0049] In response to address signals A1-AN that are provided on addresslines 130 during read, write, and erase operations, address buffers 135control the operations of the X gate decoder 115, the Y source/draindecoder 120, and the X source/drain decoder 125. The address signalsA1-AN are provided by a controller such as the control unit 30 that isfabricated separately or together with the memory 100, or otherwiseprovided by other suitable circuits. As described in detail below, theaddress signals A1-AN are decided by the X gate decoder 115, the Ysource/drain decoder 120, and the X source/drain decoder 125 to performthe reading, the writing, and the erasing operations on cells thatinclude a number of floating gate field-effect transistors (FETs) formedon the sides of a semiconductor pillar on a substrate.

[0050]FIG. 3 is a block diagram of a field programmable logic array(FPLA) 140. The FPLA 140 can be implemented to perform many of the logicfunctions performed by the ALU 20, the control unit 30 and the I/Odevices 50 as well as the X gate decoder 115, the Y source/drain decoder120 and the X source/drain decoder 125. The decoders 115, 120, 125 canbe easily reprogrammed when implemented with FPLA-like structures,thereby facilitating the addition of redundant circuit elements in thememory cell arrays 105 to improve yield.

[0051] FPLAs are commercially available standard chips that areprogrammed to perform desired logic combinations. FPLAs provide varyingnumbers of inputs and outputs and product terms, depending on theparticular logic function chosen. Arbitrary logic functions may berealized in the “sum-of-products” form that is well known to one skilledin the art. A logic function sum-of-products may be implemented usingany of the equivalent two-level logic configurations: AND-OR, NAND-NAND,NOR-OR, OR-NOR, AND-NOR, NAND-AND or OR-AND.

[0052] In the embodiment of FIG. 3, the two major constituents are anAND plane 160 and an OR plane 170. Inputs on input lines 165 aresupplied to a set of inverters and drivers 190 coupled to the AND plane160. The set of inverters and drivers 190 supply true and complementedvalues of the inputs to the AND plane 160. The AND plane 160 computesconjunctions of relevant combinations of inputs and their complements.The conjunctions and their complements are provided to the OR plane 170via interconnection lines 162. The OR plane 170 computes disjunctions ofterms applied from the AND plane 160. The disjunctions are provided asoutputs on output lines 175.

[0053] In addition, various control circuits and signals not detailedherein initiate and synchronize operations of the FPLA 140 as known tothose skilled in the art. The description of the FPLA 140 has beensimplified for purposes of illustrating the present invention and is notintended to be a complete description of all the features of a FPLA. TheFPLA 140 described herein is illustrative only and is not intended to beexclusive or limiting.

[0054]FIG. 4 is a schematic diagram illustrating one embodiment of aFPLA 141. It is well known to one skilled in the art that the FPLA 141is essentially a systematic way to implement a sum-of-products form ofone or more logical functions of a set of input variables. An arbitrarycombination logic function can be realized using the sum-of-productsform. For example, a sum-of-products may be implemented by using a twolevel logic configuration such as programmable NOR-NOR arrays 160, 170shown in FIG. 4. Each of the arrays 160, 170 is implemented withtransistors 200 having floating gates 202.

[0055] The FPLA 141 is made up of logic cells 205, 215. For the firstarray 160, the transistors 200 are arranged in cells 205AA, 205BA . . .205NA in a first dimension, e.g., in a Y-dimension of first array inputlines C1-CN, and in cells 205AA, 205AB . . . 205AN in a seconddimension, e.g., in an X-dimension of first array output lines OL1-OLN.Each of the logic cells 205 thus includes a transistor 200 having afloating gate 202 and one of the first array input lines C1-CN forreceiving input signals. The first array input lines C1-CN are alsoreferred to as first control lines 210.

[0056] In a similar manner, the second array 170 includes transistors200 arranged in cells 215AA, 215AB . . . 215AN in a first dimension,e.g., in the X-dimension of the second array input lines OL1-OLN, and incells 215AA, 215BA . . . 215NA in a second dimension, e.g., in theY-dimension of second array output lines B1-BN. Each of the logic cells215 thus includes a transistor 200 having a floating gate 202 and one ofthe first array output lines OL1-OLN. The first array output linesOL1-OLN serve as the second array input lines OL1-OLN. The second arrayinput lines OL1-OLN are also referred to as second control lines 220.Because of the substantially identical nature of logic cells 205 and215, only logic cells 205 are discussed in detail in the followingparagraphs.

[0057] The transistors 200 are floating gate transistors, which are FETshaving electrically isolated (i.e., floating) gates 202 that controlelectrical conduction between the sources S1-SN and drains 232. Thedrains 232 are interconnected with the first array output lines OL1-OLN.The sources S1-SN share a common ground for operation of the FPLA 141 inone embodiment.

[0058] As an alternative embodiment, the sources S1-SN do not share thecommon ground. The sources S1-SN are isolated from a substrate.Separation of the sources S1-SN from each other allows the FPLA 141 tobe field programmed, or erased and reprogrammed, to accomplish therequired logic functions.

[0059] The FPLA 141 has first array input lines C1-CN and second arrayoutput lines B1-BN. An N-input FPLA reaches a limiting case when it has2^(N) product terms. In the limiting case, the FPLA 141 is equivalent toa read only memory (ROM) with N address bits, and the first array 160would be identified as a ROM address decoder. However, the FPLA 141finds most effective use as a replacement for logic gates when thenumber of product terms is much smaller than 2^(N). Such a requirementis often found in the control unit 30 of the computer 10, as illustratedin FIG. 1. For example, a 32-bit very large scale integration (VLS1)computer instruction decoding unit uses a FPLA of 26 inputs, 206 productterms and 22 outputs for decoding instruction operation codes. A ROMwith 26 input bits would have more than 67 million addresses, comparedto only 206 product terms used in this example.

[0060] Programmability of the transistors 200 is achieved by chargingthe floating gates 202. When the floating gates 202 are charged, therespective transistors 200 remain in an off state until it isreprogrammed. Applying and removing charges to the floating gates 202 isdiscussed in more detail within the following paragraphs. Thetransistors 200 in an off state are represented by dotted circles 230instead of actually displaying the full transistors. The transistors 200programmed in an off state remains in that state until the charges areremoved from the floating gates 202.

[0061] The transistors 200 not having corresponding floating gates 202charged are fully illustrated in FIG. 4. These transistors 200 operatein either an on state or an off state, wherein the input signalsreceived by the first array input lines C1-CN determine the applicablestates. Each of the transistors 200 has a source and drain fabricatedusing a semiconductor pillar on a substrate. In one embodiment, thesources S1-SN are connected to the common ground for all the transistors200, and the drains 232 are the first array output lines OL1-OLN.

[0062] If any of the transistors 200 is turned on, then a ground isprovided to the source of a first pull up transistor 240. The source ofeach of the first pull up transistor 240 is connected to a respectivefirst array output line OL1-OLN. Therefore, a low voltage level isprovided on the first array output line OL1-OLN when any one of theassociated transistors 200 is activated. A high voltage level isprovided on the first array output line when the associated transistors200 are off, and the corresponding first pull up transistor 240 isturned on by a clock signal applied to a first pull up input 241, e.g.,a gate of the first pull up transistor 240.

[0063] In a similar fashion, if the transistors 200 in the second array170 are turned on via the first array output lines OL1-OLN, then aground is provided to the source of a second pull up transistor 243. Thesource of each of the second pull up transistor 243 is connected to therespective second array output line B1-BN. Therefore, a low voltagelevel is provided on the second array output line B1-BN when any one ofthe associated transistors 200 is activated. A high voltage level isprovided on the second array output line B1-BN when the associatedtransistors 200 are off, and the corresponding second pull up transistor243 is turned on by a clock signal applied to a second pull up input244, e.g., a gate of the second pull up transistor 243.

[0064]FIG. 5 is a schematic diagram illustrating one embodiment of aprogrammable decoder array 116, e.g., the X gate decoder 115. Thearchitecture of the other programmable decoders 120, 125 issubstantially similar and is not discussed in detail. The programmabledecoder array 116 of FIG. 5 is implemented with a plurality oftransistors 200, each having a corresponding floating gate 202.

[0065] In one embodiment, the programmable decoder array 116 isprogrammed at memory test to select functional wordlines within thememory cell array 105. The programmable decoder array 116 serves as anillustrative embodiment of a programmable memory address decoder using aNOR decoding scheme implemented with the transistors 200 describedherein. Other decoding schemes are acceptable, such as AND, OR, andNAND, etc.

[0066] Because of the substantially identical nature of the programmabledecoder array 116 and the first array 160 of the FPLA 141, thecharacteristics of the first array 160 discussed above applies equallyto the programmable decoder array 116. Furthermore, only the first array160 is discussed in detail in the paragraphs below with theunderstanding that the discussion is equally applicable to theprogrammable decoder array 116. One skilled in the art will readilyrecognize and understand the change in the labeling of the input andoutput lines.

[0067]FIG. 6 is a schematic diagram illustrating one embodiment of anarray 106 of memory cells, e.g., the memory cell array 105. In FIG. 6,each memory cell 265 includes two floating gate transistors 200. Thefloating gate transistors 200 are arranged in cells 265AA, 265BA . . .265NA in a first dimension, e.g., in the Y-dimension of the firstsource/drain interconnection lines YS1, YS2 . . . YSN, and in cells265AA, 265AB . . . 265AN in a second dimension, e.g., in the X-dimensionof the data lines, XD1, XD2 . . . XDN.

[0068] In the embodiment of FIG. 6, each cell 265 includes two floatinggate transistors 200 that share a common first source/drain region, suchas a source region coupled to one of the first source/draininterconnection lines YS1, YS2 . . . YSN. The floating gate transistors200 of each cell 265 also share a common second source/drain region,such as a drain region coupled to one of the data lines, XD1, XD2 . . .XDN. The first and the second source/drain regions are fabricated usinga common semiconductor pillar on a substrate, as explained below.

[0069]FIGS. 7A and 7B illustrate a top view and a front view,respectively, of a portion of an array having split control gates, i.e.two adjacent control gates 335. In the embodiment shown in FIGS. 7A and7B, the two control gates 335 overlaying corresponding floating gates202 are next to each other, as illustrated by logic cells 205AA, 205AB,205BA and 205BB. In one embodiment, the control gates 335 are coupled tothe control lines C1-CN 210 of the FPLA 141. One advantage of the splitcontrol gates 335 is that only one transistor 200 is required per logiccell 205. Each of the transistors 200 can be individually selected whenthe split control gates 335 are isolated from each other.

[0070]FIGS. 8A and 8B illustrate a top view and a front view,respectively, of a portion of an array having a single control gate 335.In the embodiment shown in FIGS. 8A and 8B, the single control gate 335overlaying corresponding floating gates 202 is in the middle of logiccells 205AA and 205BA. When the single control gate 335 is high, thetransistors 200 on both sides of the control gate 335 are activated. Inone embodiment, the control gate 335 is coupled to one of the controllines C1-CN 210 of the FPLA 141. One advantage of the single controlgate 335 is redundancy in the selection of an output, but one drawbackis the loss of circuit density because of the duplication.

[0071]FIG. 9A is a perspective view of one embodiment of verticaltransistors 200 with horizontal gate layers 202, 335, illustrated by aportion of an array having split control gates 335. For example, theembodiment of FIG. 9A is a portion of the completed FPLA 141, includingfour transistors 200 having split control gates 335, as illustrated inFIGS. 7A and 7B.

[0072] In FIG. 9A, the substantially identical transistors 200 of thecompleted FPLA 141 are illustrated by way of example through logic cells205AA, 205AB, 205BA and 205BB. Each logic cell 205 includes asemiconductor pillar 300, initially of a first conductivity type such asP− silicon, fabricated upon a monolithic substrate 305. In oneembodiment, the substrate 305 is a bulk semiconductor, such as P−silicon. In another embodiment, the substrate 305 is asemiconductor-on-insulator (SOI) substrate including an insulatinglayer, such as silicon dioxide (SiO₂), as described below.

[0073] The pillar 300 provides a source region 310, a drain region 315,and a body region 320 for the floating gate transistor 200 of aparticular logic cell, e.g., 205AA. The source region 310 is formedproximally to a sub-micron dimensional interface between the pillar 300and the substrate 305 from a second conductivity type, such as N+silicon. The drain region 315 is formed distal to the substrate 305 fromthe second conductivity type, such as N+ silicon, and separated from thesource region 310 by the body region 320 formed from the firstconductivity type, such as P− silicon.

[0074] First source interconnection lines S1-SN electricallyinterconnect the source regions 310 of pillars 300 in the Y-dimension.In one embodiment, the first source interconnection lines S1-SN comprisea conductively doped semiconductor of the second conductivity type, suchas N+ silicon, disposed at least partially within the substrate 305. Forexample, dopants can be ion-implanted or diffused into the substrate 305to form the first source interconnection lines S1-SN.

[0075] In another embodiment, the first source interconnection linesS1-SN are formed above the substrate 305. For example, a doped epitaxialsemiconductor layer can be grown on the substrate 305, from which thefirst source interconnection lines S1-SN are formed. Alternatively, anundoped epitaxial semiconductor layer can be grown on the substrate 305,and dopants then introduced by ion-implantation or diffusion to obtainthe first source interconnection lines S1-SN of the desiredconductivity.

[0076] The drain regions 315 of the pillars 300 are interconnect byoutput lines OL1-OLN in the X-dimension. FIG. 9A illustrates, by way ofexample, the output lines OL1 and OL2, which are shown schematically forclarity. However, it is understood that the output lines OL1-OLNcomprise metal or other interconnection lines that are isolated from theunderlying topology, e.g., the pillars 300, by an insulating layerthrough which contact holes are etched to access the drain regions 315of the pillars 300.

[0077] The pillar 300 is outwardly formed from the substrate 305, and isillustrated in FIG. 9A as extending vertically upward from the substrate305. The pillar 300 has a top region that is separated from thesubstrate 305 by four surrounding side regions. Floating gates 202 areformed substantially adjacent to two opposing side surfaces of thepillar 300 and separated therefrom by a gate dielectric 330, such thatthere are two floating gates 200 per pillar, though FIG. 9A omits someof the floating gates 202 for clarity of illustration.

[0078] Each of the floating gates 202 has a corresponding substantiallyoverlying control gate 335 from which it is separated by an intergatedielectric 341. Except at the periphery, there are two floating gates202 with the corresponding overlying control gates 335 interposedbetween two approximately adjacent pillars 300. For example, in FIG. 9A,control gates 335 coupled to respective control lines C1 and C2 areinterposed between the logic cells 205AA, 205BA on one side and thelogic cells 205AB, 205BB on the other side. The control line C1 controlsthe logic cells 205AA, 205BA while the control line C2 controls thelogic cells 205AB, 205BB. The adjacent control gates 335 and theircorresponding underlying floating gates 202 are separated by anintergate dielectric 340.

[0079] In this embodiment, the control gates 335 are coupled to controllines running in the Y-dimension, e.g., perpendicular to the outputlines OL1-OLN. Each of the control lines C1-CN interconnects a pluralityof transistors in the Y-dimension. For example, the control line C1electrically interconnects the control gates 335 of the logic cells205AA-205NA. The split control gates 335 allow the underlying floatinggates 202 to be independent between pairs of logic cells, 205AA and205AB, 205BA and 205BB . . . 205NA and 205BN. In the embodiment of FIG.9A, the control lines are disposed above the floating gates 202, asdescribed below.

[0080] In one embodiment, hot electron injection is used to program thefloating gate transistors 200. For example, a voltage of approximatelyzero volt is provided through one of the source interconnection linesS1-SN to the source region 310 of a particular floating gate transistor200. A voltage of approximately 5 volts is provided through one of theoutput lines OL1-OLN to the drain region 315 of the particular floatinggate transistor 200. A voltage of approximately 10 volts is providedthrough one of the control lines C1-CN to the control gate 335 of theparticular floating gate transistor 200. A resulting inversion region,i.e., channel, is formed in the body region 320 of the particularfloating gate transistor 200. Hot electrons generated in the channel areinjected through the gate dielectric 330 and onto the adjacent floatinggate 202 beneath the control gate 335.

[0081] The exact value of the voltages provided to the drain regions 315and the control gates 335 depend on the physical dimension of thefloating gate transistors 200, including the thickness of the gatedielectric 330, the thickness of the intergate dielectric 341, and theseparation distance between the source region 310 and the drain region315. Alternatively, the floating gate transistors 200 can be programmedby Fowler-Nordheim tunneling by providing higher voltages to the controlgates 335 and a thinner gate dielectric 330 and a thinner intergatedielectric 341. Electrons are tunneled from the body region 320, thesource region 310 and the drain region 315 to the floating gate 202.

[0082] The absence or presence of stored charge on the floating gate 202of a particular floating gate transistor 200 is determined by theconductivity state of the floating gate transistor 200 between itssource region 310 and drain region 315. The conductivity state isdetected by addressing one of the control lines C1-CN and one of thesource interconnection lines S1-SN to form a coincidence at theparticular floating gate transistor 200. For example, a voltage ofapproximately zero volt is provided through one of the sourceinterconnection lines S1-SN to the source region 310 of the particularfloating gate transistor 200. A positive voltage of approximately threeto five volts is provided through one of the control lines C1-CN to thecontrol gate 335 of the particular floating gate transistor 200. One ofthe output lines OL1-OLN that is coupled to the drain region 315 of theparticular floating gate transistor 200 is precharged to a positivevoltage by a pull up transistor 240.

[0083] If there are no electrons stored on the floating gate 202, thefloating gate transistor 200 conducts between its source region 310 anddrain region 315, decreasing the voltage of the corresponding outputline toward the voltage of the source region 310, e.g., toward a “low”binary logic level voltage of approximately zero volt. If there areelectrons stored on the floating gate 202, the floating gate transistor200 does not conduct between it source region 310 and drain region 315.As a result, the pull up transistor 240 tend to increase the voltage ofthe corresponding output line toward a positive voltage, e.g., toward a“high” binary logic voltage level.

[0084] In the manner described above, the floating gate transistors 200can be programmed. The programming can also be reversed, e.g., erased.In one embodiment, erasure includes providing an erasure voltagedifference between 10 and 12 volts from the source region 310 to thecorresponding control gate 335. For example, a negative voltage isprovided to the control gate 335 while a positive bias is provided tothe source region 310 to yield the erasure voltage. Alternately, zerovolt is provided to the source region 310 while a negative voltage isprovided to the control gate 335 to yield the erasure voltage. The exactvalue of the erasure voltage varies depending upon the physicaldimensions of the floating gate transistor 200, such as the thickness ofthe gate dielectric 330 and the thickness of the intergate dielectric341. Electrons are removed from the corresponding floating gate 202 byFowler-Nordheim tunneling as a result of the erasure voltage.

[0085] In one embodiment, an entire row of floating gate transistors 200is simultaneously erased by applying approximately between −10 to −12volts to one of the control lines C1-CN and also applying zero volt toone of the source interconnection lines S1-SN. In another embodiment,one or more rows of floating gate transistors 200 are simultaneouslyerased by applying approximately between −10 to −12 volts to one or morecontrol lines C1-CN and also applying zero volt to one or more sourceinterconnection lines S1-SN.

[0086]FIG. 9B is a cross-sectional view of the array of FIG. 9A lookingperpendicular to the output lines OL1-OLN, e.g., in the Y-dimension.FIG. 9B illustrates a row of logic cells 205AA, 205AB . . . 205AN,having source regions 310 interconnected by one of the sourceinterconnection lines S1, S2 . . . SN. In one embodiment, the sourceinterconnection lines are connected to ground.

[0087]FIG. 10 is a plan view looking toward the working surface of asubstrate 305, illustrating one embodiment of cells, e.g. logic cells205AB and 205AC. Two control gates 335 with corresponding underlyingfloating gates 202 are adjacent to two opposing sides of a pillar 300and separated therefrom by a gate dielectric 330. Each of the controlgates 335 is separated from its corresponding underlying floating gate202 by an intergate dielectric 341. The adjacent control gates 335 andthe adjacent floating gates 202 are likewise separated by an intergatedielectric 340. The control gates 335 are integrally formed togetherwith respective control lines C1-CN 210.

[0088] The center-to-center spacing (“pitch”) between the control gates335 that are on opposite sides of the pillar 300 is twice the minimumlithographic feature size F. An area of 4F² contains two floating gatetransistors 200, thereby implementing two logic cells 205AB and 205AC.Thus, each cell occupies an effective area of 2F².

[0089]FIG. 11 is a perspective view of an alternate embodiment ofvertical transistors with horizontal gate layers 335, 202, illustratedby a portion of an array having a single control gate 335. For example,the embodiment of FIG. 11 is a portion of the completed FPLA 141,including two logic cells 205AA and 205BA, as illustrated in FIGS. 8Aand 8B.

[0090] In the single control gate embodiment, the single control gate335 controls the two transistors 200 on opposite sides of the controlgate 335. Each of the logic cells 205 has two transistors 200 operatedfrom a common control gate 335, e.g., a single control line C1 210. Thisis in contrast to the split control gate embodiment where only onetransistor 200 is operated by a single control line 210.

[0091]FIG. 12 is a perspective view of another embodiment of verticaltransistors with horizontal gate layers, illustrated by a portion of anarray having a single control gate 335 overlaying split floating gates202. For example, the embodiment of FIG. 12 is a portion of thecompleted memory cell array 106, including four logic cells 265AA,265AB, 265BA and 265BB.

[0092] The source regions of the logic cells 265AA and 265BA areconnected by a first source/drain interconnection line YS1, while thesource regions of logic cells 265AB and 265BB are connected by a firstsource/drain interconnection line YS2. The drain regions of the logiccells 265AA and 265AB are connected by second interconnection line XD1,while the drain regions of logic cells 265BA and 286BB are connected bysecond interconnection line XD2.

[0093] In the embodiment of FIG. 12, the single control gate 335overlays the split floating gates 202. The single control gate 335 runsalong the X dimension. The split floating gates 202 allow the overlyingsingle control gate 335 to separately control the transistors 200 oneither sides of the control gate 335, thereby controlling adjacent logiccells of the memory cell array 106.

[0094] FIGS. 13-19 illustrate one embodiment of a method of formingvertical body transistors 200 with horizontal gate layers 202, 253 inthe memory cell array 106 partially shown in FIG. 12. A siliconsemiconductor substrate is oxidized and then pillars of oxide are etchedinto the substrate. Layers of oxide and silicon nitride are deposited toact as the etch mask for forming the pillars and later as a CMP etchstop. The pillars can be formed on different types of substrates,including lightly doped P-type substrate, silicon on insulatorsubstrates (e.g., SIMOX), and islands of silicon on insulator.

[0095] In this embodiment, the memory cell array 106 is formed usingbulk silicon processing techniques and is described, by way of example,with respect to a particular technology having a minimum lithographicfeature size F, which is also sometimes referred to as a criticaldimension (CD), of 0.4 microns. However, the process steps describedbelow can be scaled accordingly for other minimum feature sizes withoutdeparting from the scope of the invention.

[0096] The term “horizontal” as used in this application is defined as aplane parallel to the conventional plane or surface of a wafer orsubstrate, regardless of the orientation of the wafer or substrate. Theterm “vertical” refers to a dimension perpendicular to the horizontalplane as defined above. Prepositions, such as “on,” “side” as insidewall, “higher,” “lower,” “over,” and “under” are defined withrespect to the conventional plane or surface being the top surface ofthe wafer or substrate, regardless of the orientation of the wafer orsubstrate.

[0097]FIG. 13 illustrates a side view of a plurality of source/drainlayers on top of a substrate material 305, which serve as initialmaterials for forming vertical transistors 200 in the memory cell array106. A P− silicon starting material is used for the substrate 305. Asource layer 310, of approximate thickness between 0.2 microns and 0.5microns, forms at a working surface of the substrate 305. In oneembodiment, the source layer 310 is N+ silicon formed byion-implantation of donor dopants into the substrate 305. In anotherembodiment, the source layer 310 is N+ silicon is formed by epitaxialgrowth of silicon upon the substrate 305. A semiconductor epitaxiallayer 320, such as P− silicon of 0.6 microns approximate thickness,forms (e.g., by expitaxial growth) on top of the source layer 310.

[0098] A drain layer 315, such as N+ silicon of 150 nanometersapproximate thickness, forms at the surface of the epitaxial layer 320.The drain layer forms by ion-implantation of donor dopants into the P−epitaxial layer 320 or by epitaxial growth of N+ silicon on the P−epitaxial layer 320. A thin layer of silicon dioxide (SiO₂), referred toas pad oxide 515, is deposited on top of the drain layer 315. The padoxide 515 is approximately 10 nanometers thick. A layer of siliconnitride (SiN₄), referred to as pad nitride 520, is deposited on top ofthe pad oxide 515. The pad nitride 520 is approximately 100 nanometersthick.

[0099]FIG. 14 illustrates a plurality of first troughs 600 extendingthrough a plurality of layers 520, 515, 315, 320, 310 formed on top ofthe substrate material 305. Photoresist masking and selective etchingtechniques are used to form the plurality of substantially parallelfirst troughs 600 in the first dimension, e.g., along the Y-dimensionwhich is perpendicular to the plane of the drawing of FIG. 14. The firsttroughs 600 extend through the underlying pad nitride 520, theunderlying pad oxide 515, the underlying drain layer 315, the underlyingepitaxial layer 320, the underlying source layer 310, and at leastpartially into the underlying P− silicon substrate 305.

[0100] Conventional photoresist stripping techniques remove photoresistafter formation of the first troughs 600. Then, insulation material 605fills the first troughs 600. In one embodiment, the insulation material605 is silicon dioxide deposited by chemical vapor deposition (CVD),covering the first troughs 600 and the surrounding surfaces. Theinsulator material 605 is planarized, e.g., removed from the surfacessurrounding the first troughs 600 to expose underlying portions of thepad nitride 520, such as by chemical mechanical polishing (CMP) or othersuitable planarization technique. A masking layer 610, such as siliconnitride deposited by CVD, of approximately 200 nanometers thick forms onthe insulator 605 and elsewhere on the working surface of the substrate305. A photoresist layer 615 forms on top of the masking layer 610.

[0101]FIG. 15 is a perspective view of a plurality of second troughs 700orthogonal to the first troughs 600 of FIG. 14, further illustratingselective etching. Photoresist masking and selective etching techniquesare used to form a plurality of substantially parallel second troughs700 in the second dimension, e.g., along the X-dimension that issubstantially perpendicular to the Y-dimension. Formation of the secondtroughs 700 includes patterning the photoresist layer 615 andselectively etching the masking layer 610 along with the underlying padnitride 520 and the underlying pad oxide 515 to expose portions of thesilicon dioxide insulation 605 and the N+ drain layer 315 in the secondtroughs 700. Then, a selective etch which preferentially removes siliconbut does not substantially removes silicon dioxide etches through theexposed portions of the drain layer 315, the underlying portions of theepitaxial layer 320, and approximately 100 nanometers into theunderlying portions of the source layer 310. Conventional photoresiststripping techniques remove the photoresist 615 resulting in thestructure illustrated in FIG. 15. FIG. 15 illustrates pillars which formthe vertical bodies of the transistors 200 and trenches for gate layers.

[0102]FIG. 16 is a cross-sectional view looking in the direction of thesecond troughs 700 of FIG. 15, i.e., in the X-dimension orthogonal tothe plane of the illustration. A thin oxidation barrier layer 900, suchas silicon nitride of approximately 20 nanometers thick, is conformallydeposited by CVD to mask (i.e., to protect against oxidation of) thesidewalls of the second troughs 700. The barrier layer 900 isdirectionally etched after deposit to expose bottom portions of thesecond troughs 700, leaving the silicon nitride on the sidewalls.

[0103] Bottom insulation layers 905, high quality oxide for isolation,are formed on the bottoms of the second troughs 700 to insulateinterconnection lines connecting the source regions 310 of thetransistors 200, e.g., the first source/drain interconnection lines YS1,YS2 . . . YSN in the memory cell array 106. In one embodiment, thebottom insulation layers 905 are silicon dioxide formed by thermaloxidation of the exposed bottom portions of the second troughs 700. Thesilicon dioxide has a thickness of approximately 100 nanometers.

[0104]FIG. 17 is a cross-sectional view looking the direction of thesecond troughs 700 of FIG. 16, illustrating the barrier layer 900stripped from the sidewalls of the second troughs 700. In FIG. 17, briefphosphoric acid etch strips the barrier layers 900 from the sidewalls ofthe second troughs 700. The brief phosphoric acid etch is timed toexpose the sidewalls of the second troughs 700 without significantremoval of the thick silicon nitride masking layer 610. Gate dielectriclayers 330 form substantially adjacent to the exposed sidewalls of therespective second troughs 700. In one embodiment, the gate dielectriclayers 330 are silicon dioxide, sometimes referred to as “tunnel oxide,”with a thickness approximately between 4 nanometers and 8 nanometers.Conductive layers 1005, such as N+ doped polysilicon, form by CVD tofill the respective second troughs 700. The conductive layers 1005 areplanarized by CMP or other suitable planarization techniques.

[0105]FIG. 18 is a cross-sectional view looking in the direction of thesecond troughs 700 of FIG. 17, illustrating a conductive layer in thesecond troughs. In FIG. 18, the conductive layers 1005 are etched backin the second troughs 700 to approximately at or slightly above thelevel of the silicon surface which is defined by the interface betweenthe drain layers 315 and the pad oxides 515. A spacer layer, such assilicon nitride approximately 100 nanometers in thickness, is depositedby CVD and directionally etched by reactive ion etching (RIE) to leavenitride spacers 1100 along the sidewalls of the second troughs 700,i.e., on the etched back portions of the conductive layers 1005, onportions of the insulation 605 in the intersection of the first troughs600 and the second troughs 700, and against the gate dielectric layers330.

[0106]FIG. 19 is a perspective view of the second troughs 700 of FIG.18, illustrating the spacers 1100 positioned for use as a mask. Thespacers 1100 are masks for anisotropic etching in the second troughs 700of the etched back portions of the polysilicon conductive layer 1005 andportions of the silicon dioxide insulation 605. A selective etch whichremoves silicon dioxide without substantially removing polysiliconetches into portions of the silicon dioxide insulation 605 withoutdisturbing the portions of the polysilicon conductive layers 1005 in thesecond troughs 700.

[0107] The portions of the silicon dioxide insulation 605 in the secondtroughs 700 are etched until they are approximately even with adjacentportions of the bottom insulation layer 905. Then, a selective etchwhich removes polysilicon without substantially removing silicon dioxideetches through portions of the conductive layers 1005 in the secondtroughs until the bottom insulation layers 905 is exposed, therebyforming separate floating gates 202 along the sidewalls of the secondtroughs 700.

[0108] Referring back to FIG. 12, the intergate dielectric 340 is formedin the second troughs 700 by filling the space between the floatinggates 202 with deposited oxide. The oxide is deposited and planarized tothe level of the masking layer 610. Then, the oxide is etched back toapproximately the level defined by the interface between the drainlayers 315 and the pad oxides 515. Next, silicon nitride (e.g., from themasking layer 610, the pad nitride 520 and the spacers 1100) is removed.In one embodiment, phosphoric acid is used to remove the siliconnitride.

[0109] The intergate dielectric 341 is formed by growing thermal controlgate oxide or depositing oxynitride approximately 8 to 20 nanometers inthickness above the floating gates 202. The control gate 335 ofapproximately 200 nanometers thick is formed by depositing suitable gatematerial, such as N+ doped polysilicon, on top of the floating gates202. In one embodiment, the N+ doped polysilicon is deposited by CVD andpatterned into horizontal bars above the floating gates 202. Finally, anoxide layer (not shown), deposited by CVD, covers the working surfacefor the subsequent steps of creating contact holes, terminal metal, andinter level insulation to complete wiring of the cells and peripheralcircuits.

[0110]FIG. 20 is a perspective view of the second troughs 700 of FIG.17, illustrating an alternate method of forming vertical transistors 200with horizontal gate layers 335, 202. In FIG. 20, the conductive layers1005 are etched back in the second troughs 700 to approximately at orslightly above the level of the silicon surface which is defined by theinterface between the drain layers 315 and the pad oxides 515, therebyforming floating gates 202. Then intergate dielectric 341 is formed bygrowing thermal control gate oxide or depositing oxynitrideapproximately 8 to 20 nanometers in thickness above the floating gates202. Control gates 335 of approximately 200 nanometers thick are formedby depositing and planarizing suitable gate material, such as N+ dopedpolysilicon, on top of the intergate dielectric 341. In one embodiment,the N+ doped polysilicon is deposited by CVD and patterned intohorizontal bars above the floating gates 202, resulting in the verticaltransistors 200 of FIG. 11 with single control gates 335.

[0111]FIG. 21 is a cross-sectional view looking in the direction of thesecond troughs 700 of FIG. 16, illustrating yet another method offorming vertical transistors 200 with horizontal gate layers 335, 202.The barrier layers 900 of FIG. 16 are stripped from the sidewalls of thesecond troughs 700 using brief phosphoric acid.

[0112] A first layer 1200 is deposited, planarized and etched back inthe second troughs 700 to approximately at or above the level of thesurface which is defined by the interface between the source region 310and the body region 320. A second layer 1202 is then deposited on top ofthe first layer 1200, planarized, and etched back in the second troughs700 to approximately at or below the level of the surface which isdefined by the interface between the body region 320 and the drainregion 315. Finally, a third layer 1204 is deposited on top of thesecond layer 1202 and planarized.

[0113] In one embodiment, the first layer 1200 and the third layer 1204are doped materials, such as N+ doped oxide. The second layer 1202 is anundoped material, such as undoped oxide. In an alternate embodiment, thesecond layer 1202 is undoped polysilicon.

[0114] The structure of FIG. 21 is then heat treated to form lightlydoped source/drain regions 1300 shown in FIG. 22. The lightly dopedsource/drain regions 1300 are in the vicinity of the contact surfacesbetween the first layer 1200 or the third layer 1204 and the body region320. Diffusion of dopants in the first layer 1200 and the third layer1204 during the heat treatment results in the lightly doped source/drainregions 1300 in the body region 320.

[0115]FIG. 22 is a cross-sectional view looking in the direction of thesecond troughs of FIG. 21, illustrating gate length control andself-aligned transistor structures. In one embodiment, the second layer1202 is a sacrificial gate layer which defines a gate length of thetransistor 200. The sacrificial gate layer 1202 is selectively removedand replaced with a floating gate 202 in a subsequent step.

[0116] For example, the third layer 1204 is removed after the beattreatment that produces the lightly doped source/drain regions 1300.Gate dielectric layers 330, such as gate oxide, are grown substantiallyadjacent to the exposed sidewalls of the respective second troughs 700.The sacrificial gate layer 1202 is then selectively removed and replacedwith a conductive layer 1005, such as doped polysilicon.

[0117] Based on the structure of FIG. 22, techniques described above canbe used to form various embodiments of the vertical body transistors200. For example, techniques shown in FIGS. 18 and 19 are used to formvertical body transistors 200 with split floating gates. Techniquesshown in FIG. 20 are used to form vertical body transistors 200 with asingle floating gate.

[0118] The lightly doped source/drain regions 1300 act as extensions ofthe source and drain regions 310, 315. The lightly doped source/drainregions 1300 reduce the channel or gate length which is the distancenext to the floating gate 202 in the body region 320 between the sourceregion 310 and the drain region 315. The thickness of the second layer1202 partially controls the channel length during the heat treatmentdescribed in FIG. 21. When the floating gate 202 replaces the secondlayer 1202, minimal gate overlaps occur with the lightly dopedsource/drain regions.

[0119] Though FIGS. 13-22 illustrate various embodiments of forming thevertical body transistors 200 using bulk silicon processing techniques,other processing techniques can be used. In one embodiment, asemiconductor-on-insulator (SOI) substrate is formed from a substrate305. For example, a P− silicon starting material is used for thesubstrate 305, and processing begins similarly to the bulk semiconductorembodiment described in FIG. 13. However, after the first troughs 600are formed in FIG. 14, an oxidation barrier layer is formed on thesidewalls of the first troughs 600. An isotropic chemical etch is usedto fully undercut the semiconductor regions separating the first troughs600, and a subsequent oxidation step is used to fill the evacuatedregions formed by the undercutting. As a result, an insulator is formedon the bottoms of the first troughs 600, bars of SOI are formed betweenfirst troughs 600, and the topography on the working surface of thesubstrate 305 is separated from the substrate 305 by an insulatinglayer. The barrier layer is then removed from the sidewalls of the firsttroughs 600, which are then filled with insulator 605, as illustrated inFIG. 14.

[0120] Thus, in the above described figures, the substrate 305 isunderstood to include bulk semiconductor as well as SOI embodiments inwhich semiconductor integrated circuits formed on the surface of thesubstrate 305 are isolated from each other and an underlyingsemiconductor portion of the substrate 305 by an insulating layer. Onesuch method of forming regions of SOI is described in U.S. Pat. No.5,691,230 issued to Forbes and assigned to the assignee of the presentapplication, and which is herein incorporated by reference.

[0121] A common problem among the programmable memories and the relatedFPLAs is a large cell size, which limits circuit density. The cell sizecan be decreased, thereby increasing the circuit density, by introducinga novel physical structure for the floating gate transistors.

[0122] The present invention provides a novel structure for floatinggate transistors 200 which improves circuit density. For example, if afloating gate transistor 200 is used to store a single bit of data inthe memory cell array 106, an area of only 2F² is needed per bit ofdata. If multiple charge states (more than two) are used, even less areais needed per bit of data, e.g., an area of F² is needed per bit of datawhen four charge states are used. The increased storage capacity of thememory cell array 106 is particularly advantageous in replacing harddisk drive data storage in a computer 10. In such an application, thedelicate mechanical components included in the hard disk drive arereplaced by rugged, small, and durable solid-state high density memorycell arrays 106, such as EEPROMs. For example, high density flashEEPROMs provide improved performance, extended rewrite cycles, increaseddurability, lower power consumption, and improved portability.

[0123] Although described above in connection with particularembodiments of the present invention, it should be understood that thedescriptions of the embodiments are illustrative of the invention andare not intended to be limiting. Various modifications and applicationsmay occur to those skilled in the art without departing from the truespirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. A method for forming an array of verticaltransistors with horizontal gate layers, said method comprising: formingsource, body and drain layers on top of one another on a substrate;etching the source, body and drain layers to form substantially parallelfirst troughs in a first dimension and substantially parallel secondtroughs in a second dimension which is substantially orthogonal to thefirst dimension; forming floating gates along sidewalls of the secondtroughs; and forming control gates on top of the respective floatinggates.
 2. A method for forming a vertical transistor with horizontalgate layers, said method comprising: forming source, body and drainlayers on top of one another on a substrate; etching the source, bodyand drain layers to form substantially parallel first troughs in a firstdimension and substantially parallel second troughs in a seconddimension which is substantially orthogonal to the first dimension;depositing dopant layers and a sacrificial gate layer in the secondtroughs; heating the partially formed array to form lightly dopedsource/drain regions in the body layer; removing at least one of thedopant layers; replacing the sacrificial gate layer with a floatinggate; and forming a control gate above the floating gate.